With an increase of integration of IC circuits, heat value is increased in electronic parts such as hybrid packages, multimodules, and sealed-type integrated circuits with plastics and metals. Since the increase of temperature due to the increased heat value may cause malfunction of the electronic parts, a countermeasure of preventing the malfunction by attaching a heat-radiating member such as a heat sink to the electronic part has been taken.
As a method for providing electronic parts with heat-radiating members, a method which comprises using an adhesive prepared by adding aluminum powder etc. to a composition comprising a polymerizable acrylate monomer and a free radical initiator is proposed in U.S. Pat. No. 4,722,960. In this method, after the adhesive is applied onto one or both of the electronic part and the heat-radiating member, it is necessary to effect curing treatment using a primer or blocking oxygen. Such an adhesion treatment needs a long time and much labor, and the materials to be adhered have to be fixed temporally until the completion of curing, thereby resulting in poor efficiency in producing the electronic devices.
Further, U.S. Pat. No. 4,606,962 proposes to use an adhesive tape which has an adhesive layer containing silver grains having a diameter exceeding the thickness of the layer. When silver grains are added in the process of the preparation of the adhesive composition, the viscosity of the composition is extremely increased and, as a result, the composition becomes poor in flowability and handling properties, in particular, coating workability. When such a composition is formed into a tape, no thickness accuracy can be achieved, and in the worst case, no tape can be obtained. In addition, there arises another problem that the tape thus obtained is unusable in the case where electrical insulation is needed.
In contrast, there have been proposed some methods which comprise using pressure-sensitive adhesive tapes containing heat-conductive and electrical insulating particles in the adhesive. When inserted between electronic parts to be electrically insulated and heat-radiating members, these adhesive tapes can adhere and fix them utilizing the pressure-sensitive adhesion without showing any trouble in handling properties, etc., which the adhesive tapes containing silver grains cause.
In some of these heat-conductive and pressure-sensitive adhesive tapes, those which do not need any base material are known, for example, as disclosed in JP-A-6-88061 corresponding to EP 566093 A1 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, in order to establish sufficient tape strength for fully adhering and fixing electronic parts to heat-radiating members, a layer of the pressure-sensitive adhesive composition containing heat-conductive and electrical insulating particles is preferably formed on a highly heat-resistant plastic film as the base material.
As the heat-conductive and pressure-sensitive adhesive tape having the above-mentioned constitution and being on market, an acrylic pressure-sensitive adhesive comprising a methyl acrylate/2-ethylhexyl acrylate copolymer as the main component is known. However, tapes of this type suffer from some troubles in heat resistance, frequently undergo anchoring fracture between the base material and the adhesive layer, and are poor in the capability of adhering and fixing electronic parts to heat-radiating members.
On the other hand, as a heat-conductive interfacial material located between a heat source and a radiator, JP-A-5-198709 (U.S. Pat. No. 5,213,868) proposes a heat-conductive material comprising a plastic film (made of nylon, polyesters, polyimides, polyamides, etc.) having a pressure-sensitive acrylic adhesive layer formed thereon and perforated, embossed or grooved so as to remove the air between the heat source and the radiator. However, these materials are also unsatisfactory in heat resistance and fail to achieve any sufficient effects as an interfacial material.
Namely, electrical insulating, heat-conductive and pressure-sensitive adhesive tapes are advantageous in that electronic parts to be insulated can be easily adhered and fixed to heat-radiating members without resort to any troublesome and time-consuming adhesion treatment. However, these tapes are poor in heat resistance and frequently undergo anchoring fracture between the base material and the adhesive layer, as described above. In order to apply these tapes to fixation of electronic parts to heat-radiating members, it is strongly desired to overcome this problem.
Under these circumstances, the present invention aims at providing a heat-conductive and pressure-sensitive adhesive sheet which is excellent in heat resistance, scarcely undergoes anchoring fracture between the base material and the adhesive layer and, therefore, is usable in fixing electronic parts to heat-radiating members or fixing members in various fields including constructive materials, vehicles, aircrafts and ships.
In order to achieve the above-mentioned object, the present inventors have conducted extensive studies and, as a result, have achieved the present invention based on the finding that a heat-conductive and pressure-sensitive adhesive sheet having excellent heat resistance and improved adhesion of the adhesive layer to the base material and scarcely undergoing anchoring fracture between the base material and the adhesive layer can be obtained by using a plastic film having a specific composition, i.e., containing a heat-conductive, electrical insulating filler as the base material, subjecting one or both of the surfaces of the film to high frequency sputter etching, and then forming a heat-conductive and pressure-sensitive adhesive composition having a specific composition, i.e., containing an acrylic polymer having hydrophilic groups, on the sputter-etched surface(s).